Autoclavable electrochemical cell

ABSTRACT

An autoclavable elctrochemical cell which may be used in an implantable medical device. The anode active material is lithium or other material from groups IA and IIA of the Periodic Table and having a melting point greater than about 150 degrees C. The cathode active material is silver vanadium oxide or other metal oxide or carbon monoflouride. The solvent for the electrolyte has a boiling point greater than about 100 degrees C. and a dielectric constant greater than about 5 so that the cell may be dimensionally and chemically stable during repeated exposures of about one hour each to the autoclaving temperatures.

This application is a continuation of Ser. No. 09/750,701 filed Jan. 2,2001 , now abandoned which is a continuation of Ser. No. 09/551,830filed Apr. 18, 2000, now abandoned, which is a continuation of Ser. No.08/403,570 filed Mar. 14, 1995, now U.S. Pat. No. 6,150,057, which is acontinuation of Ser. No. 08/273,604 filed Jul. 12, 1994, now abandoned,which is a continuation of Ser. No. 07/987,584 filed Dec. 8, 1992, nowabandoned, which is a continuation of Ser. No. 07/767,855 filed Sep. 30,1991, now abandoned.

The present invention relates generally to the art of electrochemicalcells and more particularly autoclavable electrochemical cells orbatteries such as may be used, for example, in implantable medicaldevices.

Numerous power sources have been developed for use in implantabledevices such as implantable drug pumps and pacemakers. It is importantthat the medical devices be sterilized prior to implantation in thebody. Medical devices have been sterilized by treatment with an oxidegas such as ethylene oxide (filling oxide gas treatment). However, inaddition to being considered environmentally unsafe, ethylene oxide gasis necrotic to tissue. During sterilization the ethylene oxide gas maybecome trapped in spaces within a medical device with the result thatits eventual release, after implantation of the device, may lead topotentially severe tissue damage in the patient.

An alternative to ethylene oxide gas treatment is sterilization of themedical device in an autoclave. For such sterilization the implantablemedical device and the electrochemical cell which serves as its powersource must be capable of withstanding the repeated prolonged exposuresto heat soak and other autoclaving conditions at the high temperatureson the order of 130 to 135 degrees C. encountered.

Batteries for implantable medical devices may include anodes having asactive material lithium or other alkali metal, cathodes having as activematerial silver vanadium oxide or other metal oxide or carbonmonoflouride, electrolytes composed of a lithium salt and an organicsolvent, and a, separator material between the electrodes and which isporous for passage of the electrolyte therethrough for ionic transferbetween the electrodes for generating a current. Examples of suchbatteries are disclosed in U.S. Pat. Nos. 4,057,679; 4,618,548; and4,830,940. While batteries have been provided which have operatingtemperatures within the range of minus 55 to plus 225 degrees C., asdiscussed in related U.S. Pat. Nos. 4,310,609 and 4,391,729, which areassigned to the assignee of the present invention, the ability of abattery to operate in such a temperature range does not determinewhether it has the ability to withstand the heat soak and otherconditions of autoclaving at temperatures of about 130 to 135 degrees C.

Other patents which may be of interest include U.S. Pat. Nos. 4,751,157;4,751,158; 4,146,685; 4,574,113; 4,615,959; 4,668,594; 4,668,595; and4,735,875.

As discussed by the inventors of the present invention in an articleentitled “Autoclavable Li/Silver Vanadium Oxide Cell”, Progress inBatteries & Solar Cells, Volume 8 (1989), at pages 122-125, a desirablecharacteristic of some medical cells is the ability of the cells towithstand repeated high temperature excursions that occur duringsterilization in an autoclave without loss of deliverable capacity.

Such batteries as disclosed in the aforesaid patents are deficient forpurposes of autoclaving since their compositions are such that one ormore of their components may render the cell dimensionally and/orchemically unstable during repeated exposures at autoclavabletemperatures or cause a significant reduction in the cell's capacity asa consequence of such exposures.

It is accordingly an object of the present invention to provide anelectrochemical cell which can withstand repeated exposure to autoclaveenvironments without significant loss of capacity.

In order to provide such an autoclavable electrochemical cell, inaccordance with the present invention the anode is provided to have asactive material a material which has a melting point greater than about150 degrees C. and the solvent for the electrolyte is characterized byhaving a boiling point greater than about 100 degrees C. and adielectric constant greater than about 5.

The above and other objects, features, and advantages of the presentinvention will be apparent in the following detailed description of thepreferred embodiments taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing discharge under a five kilohm load of a cellof the present invention which has not undergone autoclaving.

FIG. 2 is a graph showing discharge of the cell of FIG. 1 after it hasbeen subjected to five exposures each of about one hour to anautoclaving environment at a temperature of about 130 degrees C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A battery suitable as a power source for an implantable medical deviceis comprised of a casing, cathode plates having as active materialsilver vanadium oxide (a metal oxide bronze), an anode having as activematerial lithium, a non-aqueous electrolyte solution which includes alithium salt and an organic solvent, and a separator materialencapsulating either or both of the electrodes, as discussed in greaterdetail in the aforesaid U.S. Pat. No. 4,830,940, which is assigned tothe assignee of the present invention and the disclosure of which ishereby incorporated herein by reference. Other suitable active materialsfor the anode may be selected from groups IA and IIA of the PeriodicTable. The cathode active material may suitably comprise other metaloxides, which are meant to include the metal oxide bronzes, and carbonmonoflouride. Examples of metal oxides for the cathode material include,but are not limited to, manganese dioxide, vanadium oxide, and cobaltoxide. The reference herein and in the claims to lithium is meant toinclude alloys thereof.

The exposure of such a cell as described above to elevated temperaturesduring repeated periods of autoclaving requires that the cell beconstructed so that it remains dimensionally and chemically stable.Thus, the cell suitably should retain dimensional and chemical stabilityduring repeated exposures each of about one hour to a temperature ofabout 130 to 135 degrees C. For the purposes of this specification andthe claims, the term “repeated” is meant to refer to at least five suchexposures. The term “dimensionally stable” refers to the ability of thecell to resist swelling. As the temperature increases the pressureinside the cell increases. This may result in cell swelling if a cellincludes an unsuitable electrolyte. The case walls undesirably bulgeduring such cell swelling due to high vapor pressure. The term“chemically stable” is meant to refer to maintenance of the chemicalcomposition of the cell components so that performance of the cell isnot significantly compromised by the autoclaving heat whereby it doesnot have reduced capacity or increased cell resistance or decreased celllife.

In order to minimize the generation of gas as the temperature isincreased during autoclaving for dimensional stability, the electrolytesolvent is selected to have a high boiling point, i.e., at least about100 degrees C. The solvent is also selected to have a high dielectricconstant, i.e., at least about 5 so that the cell capacity may bemaintained during the repeated exposures to an autoclaving environment.The electrolyte solvents are also selected to be thermally stable in thepresence of the electrode active materials at the autoclavingtemperature. Examples of solvents which are suitable for use with cellshaving lithium anodes and silver vanadium oxide cathodes include, butare not limited to, diglyme, sulfolane, propylene carbonate, ethylenecarbonate, and mixtures thereof.

The salt and solvent combination should be such that high conductivityis provided, high thermal stability is present, and the cell candischarge effectively at room temperature and at 37 degrees C. By“thermal stability” is meant the ability of a material not to weaken ormelt or degrade at the autoclaving temperature. This, includes theability of the salt not to precipitate out of the electrolyte solutionupon exposure to the autoclaving temperatures. Suitable lithium saltsinclude, but are not limited to, lithium tetrafluoroborate, lithiumtrifluoromethane sulfonate, lithium hexafluoroarsenate, lithiumhexafluorophosphate and lithium perchlorate.

The separator material is composed of an electrically insulativematerial to prevent an internal electrical short circuit between theelectrodes, is chemically unreactive with the electrode materials, isboth chemically unreactive with and insoluble in the electrolytesolution, and has a sufficient porosity to allow flow-through of theelectrolyte solution during the electrochemical reaction in the cell. Ifthe separator material were insufficiently wettable by the electrolytesolvent, there would be too high of resistance to flow of theelectrolyte through the pores thereof due to the increased surfacetension. In addition, melting or a tendency to melt of the separatormaterial, if it has insufficient thermal stability, may tend to clog theopenings therein to thereby undesirably prevent or reduce electrolyteflow. In order to have adequate thermal stability, the separatormaterial is chosen to have a melting point of preferably at least about130 degrees. Examples of suitable separator materials for theaforementioned electrolytes include, but are not limited to,polypropylene non-woven material, polypropylene membrane material,polypropylene laminate of non-woven and membrane material, a Teflonmembrane material in conjunction with a polypropylene non-woven layer,and halogenated polymeric membranes such as Tefzel membranes provided byScimat, Inc.

The active material of the anode, as well as that of the cathode, mustalso be thermally stable, i.e., have a melting point which is greaterthan about 150 degrees C., so that it can withstand the autoclavingtemperatures without undesirably melting or degrading.

Repeated exposure to the elevated temperatures during autoclaving canaggravate corrosion problems. In order to prevent such corrosion, thevarious metallic cell components are suitably made ofcorrosion-resistant materials such as, for example, stainless steel ortitanium. Glass seals providing feed-throughs for the electrodes aresuitably composed of corrosion resistant glass. The cathode may suitablybe enclosed in the separator material and then placed into the casecontaining the anode plates, which may also be enclosed in separatormaterial, and the cell then vacuum filled with electrolyte after which afinal close welding provides an hermetic seal of the case.

A preferred electrolyte for the cells of the present invention compriseslithium trifluoromethane sulfonate salt, having a good thermalstability, in a high conductivity solvent comprising a mixture ofpropylene carbonate and diglyme (2-methoxy ethyl ether), both havinghigh boiling points and good thermal stability as well as thecombination of solvents providing a higher conductivity than any of theaforementioned suitable solvents alone. While lithium trifluoromethanesulfonate is preferred, it should be understood that other suitablesalts may be used with the mixture of propylene carbonate and diglyme. Asuitable ratio, by volume, for the mixture of propylene carbonate anddiglyme is 50:50.

A separator material composed of a laminate of a polypropylene membraneand a polypropylene mesh is not suitably wettable by propylene carbonatealone due to the low viscosity of propylene carbonate. However, thepolypropylene laminate is sufficiently wettable by and may be used withthe combination of propylene carbonate and diglyme, wherein the diglymecomprises at least about 10 percent by volume of the mixture, since thediglyme tends to thin the mixture.

Titanium reacts with carbon monofluoride at high temperatures toincrease cell impedance. If the cathode active material is carbonmonofluoride, the current collector therefor is suitably composed ofsuperferrite or titanium coated with a carbon paint or other suitableconductor. In order to provide an electrolyte which has good stabilitywhen used with the carbon monofluoride cathode material, the electrolytetherefor is preferably selected to be lithium tetrafluoroborate orlithium trifluoromethane sulfonate in gammabutyrolactone solvent, whichhas a suitably high boiling point.

The following is an example of a cell made in accordance with thepresent invention and a comparison of its operating characteristicsbefore and after repeated exposures thereof to an autoclavingenvironment, it being understood that the following example is beingprovided for illustrative purposes only and not for purposes oflimitation.

Cells, that have a half-round shape of dimensions 7 mm×28 mm×43 mm, wereconstructed in accordance with the present invention. The cathodematerial comprised, by total weight-percent, 98% silver vanadium oxide(SVO), 1% Teflon 7A material, and 1% graphite. The anode material wascomposed of lithium, and the electrodes of the cells were prohibitedfrom coming into contact with each other by using separators composed ofGoretex polypropylene laminate material of a membrane and a non-wovenmesh, a product of W. L. Gore & Assoc. The electrolyte used wascomprised of 1M lithium trifluoromethane sulfonate as the saltcomponent, and a 1:1 mixture of propylene carbonate: diglyme as theorganic solvent component. The cell components were composed ofcorrosion-resistant materials, and the casing was hermetically sealed.The cells have a theoretical capacity of about 2.3 Ah with a volumetricenergy density of about 870 Wh/L and a gravimetric density of about 260Wh/kg.

The performance of the cells under 1 or 5 kilohm loads was observed.Cells that were subjected to five autoclave cycles, each cycle attaininga temperature to about 130 degrees C. for about one hour, were comparedto identical cells which were not autoclaved. The comparison, as shownin Table I, indicates that there is no significant difference indelivered capacity between cells that were autoclaved and those thatwere not. Cells discharged under 1 kilohm delivered an average of 1.93Ah or 84% of theoretical capacity to a 2 volt cutoff when they wereautoclaved compared to 1.93 Ah or 84% when they were not. The 5 kilohmgroup delivered 2.06 Ah or 90% of theoretical capacity to a 2 voltcutoff when they were autoclaved and 2.07 Ah or 90% theoretical capacitywhen they were not autoclaved. Typical discharge curves of theautoclaved and non-autoclaved cells under 5 kilohm load are shown inFIGS. 1 and 2 respectively.

TABLE I Experimental to 2V % Theoretical 2V (Ah) (Ah) 1 kilohm DischargeAutoclaved 1.93 84 Non-autoclaved 1.93 84 5 kilohm Discharge Autoclaved2.06 90 Non-autoclaved 2.07 90

The discharge behavior of the cells was also observed. Theself-discharge of the cells was estimated from heat dissipated asmeasured by calorimetry. Microcalorimetry testing at 37 degrees C. wasperformed on cells stored at open circuit after 2, 5 and 8 months. At 2months, the autoclaved cells showed annual self-discharge rates of 0.6%to 0.8% while the non-autoclaved cells showed rates of 1.2% to 1.3%. At5 months there was still slightly less heat dissipation from theautoclaved cells than from non-autoclaved cells, but the difference wasnarrowed (avg. 0.55% vs. 0.67%) and after 8 months the averages were0.28% vs. 0.43% for the average annual self-discharge. Thus, aspresented in Table II, the microcalorimetry testing indicated less than1% self-discharge per year for cells of this autoclavable design.

TABLE II Microcalorimetry Test Results % Self-Discharge 2 Months 5Months 8 Months Autoclaved 0.72 0.55 0.28 Non-autoclaved 1.28 0.67 0.43

The above detailed description and examples are intended for purposes ofillustrating the invention and are not to be construed as limiting. Theinvention can be embodied otherwise without departing from the principlethereof, and such other embodiments are meant to come within the scopeof the present invention as defined by the appended claims.

What is claimed is:
 1. In an electrochemical cell comprising a casing;an anode; a solid cathode having as active material a material selectedfrom the group of materials consisting of metal oxide bronzes and carbonmonofluoride; and an ionically conductive electrolyte solution, which isoperatively associated with said anode and cathode, comprising a lithiumsalt and an organic solvent, wherein the improvement comprises acombination of components rendering the electrochemical cellautoclavable and dimensionally and chemically stable during repeatedprolonged exposures to heat of from about 130° C., said combinationcomprising: an anode having as active material a material which has amelting point greater than 150° C. and which is selected from groups IAand IIA of the Periodic Table; and a mixed electrolytic organic solventhaving a boiling point greater than about 100° C. and a dielectricconstant greater than about 5 selected from the group consisting ofsulfolane, ethylene carbonate, propylene carbonate andgammabutyrolactone.